Researchers have discovered a link between air-pollution exposure before birth and lower IQ scores in childhood.

The finding came from a study of 249 children of New York City mothers who wore backpack air monitors for 48 hours during the last few months of pregnancy. They lived in mostly low-income neighborhoods in northern Manhattan and the South Bronx and were nonsmokers.

They had varying levels of exposure to typical kinds of urban air pollution, mostly from car, bus and truck exhaust.

At age 5, before starting school, the children were given IQ tests. Those exposed to the most pollution before birth scored on average 4 to 5 points lower than children with less exposure.

That’s a big enough difference that it could affect performance in school, said Frederica Perera, the study’s lead author and director of the Columbia Center for Children’s Environmental Health.